|  | 5.4 | IGRP Operations
| 5.4.5 | IGRP metrics and routing updates
IGRP uses several types of metric information. For each path through an AS, IGRP records the segment with the lowest bandwidth, the accumulated delay, the smallest maximum transmission unit (MTU), and the reliability and load. Variables are used to weight each metric, and bandwidth is, by default, given the most importance when calculating the best path. For a network of one medium (such as a network that uses all Ethernet), this metric reduces to a hop count. For a network of mixed media (for example, Ethernet and serial lines running from 9600 baud to T1 rates), the route with the lowest metric reflects the most desirable path to a destination. A router running IGRP sends an IGRP update broadcast every 90 seconds. It declares a route inaccessible if it does not receive an update from the first router in the route within three update periods (270 seconds). After seven update periods (630 seconds), the router removes the route from the routing table. IGRP uses flash update and poison reverse to speed up the convergence of the routing protocol. A flash update is the sending of an update sooner than the standard periodic update interval of notifying other routers of a metric change. Poison reverse updates are intended to defeat larger routing loops that are caused by increases in routing metrics. The poison reverse updates are sent to remove a route and place it in holddown, which keeps new routing information from being used for a certain period of time.